In the Arab world, when a man gets married he makes a payment to his bride's family. However in parts of Yemen when a brother and sister from one family marry a brother and sister from another, dowries are often not paid - but this can have tragic consequences.

A young man approaches a friend to ask for his sister's hand in marriage - in exchange for his own sister's hand. This is "swap" marriage or "shegar" as it is known in Arabic, an ancient marriage custom still practised in Yemen.

The way it works is: "I'll marry your sister, if you marry mine."

But the other side of the bargain is: "If you divorce my sister, I'll divorce yours."

Swap marriage came about as a way to help poorer families avoid paying dowries, and that is still a big attraction to some families in Yemen today. A dowry can come to about $3,500 - even though most people earn less than $2 a day.

When there is no money to pay for the dowry and other wedding expenses, that's when "people marry shegar" says Mohamed Hamoud, a village elder in Sawan, not far from the capital Sanaa.

But the survival of swap marriage also owes something to the fact that Yemen is a deeply proud and conservative country whose strict adherence to ancient traditions and values have kept the fabric of society unchanged.

"Our traditions are too important to us," Hamoud says.

He acknowledges, though, that the practice is in decline, for one simple reason: "It causes too much misery."

That's because couples forced first to love can sometimes then be forced to divorce.

Nadia, a young woman in her late 20s, married a man whose sister married her brother. It was a happy marriage and she had three children - before her brother's marriage broke down, and she and her husband were torn apart.

"Swap marriage is the worst kind of marriage, it's better to spend all your life alone than to marry this way," she cries.

Her children were taken away from her, including her youngest, who was then seven months old.

"I begged them to return my daughter to me, I told them, 'It's not right, she needs me to breastfeed her.' I asked them, 'What have I done wrong?'"

She had done nothing wrong. For her in-laws it was simply a tit-for-tat response. What happened to their daughter had to happen to her.

Nadia considered resorting to the law to get her children back, as the law does side with mothers in these cases, but she decided against it. In practice, tribal and social customs tend to overrule the law of the state.

She did not see her daughter again for three years. "When I saw her for the first time after all those years I thought to myself, 'She won't recognise me.' I imagined her saying: 'You are not my mother how could you be my mother when I haven't seen you since I was a few months old?'" she says.

Many religious scholars oppose swap marriage and have declared it un-Islamic on the grounds that the dowry is an essential part of the Muslim marriage contract.

"The dowry payment is meant to provide women with some financial security as they leave their home," Yemeni sheikh Mohamed Mamoun explains.

Find out more

Waleed and others tell their stories in Yemen's Swap Marriages on the BBC World Service on Tuesday 29 July from 04:30 GMT

But in some cases swap marriages occur even when families do pay a dowry. In fact, whenever two families exchange daughters, the couples' fates will most likely be sealed together.

Brother and sister Waleed and Nora married their cousins in shegar, but both families paid dowry and agreed not to make the two marriages dependent on each other.

The swap in this instance was meant to ease the mounting pressure on parents to find suitors for their daughters. In a country where more than a quarter of females are married off before the age of 15, a girl's family starts to worry if their daughter is not asked for by her mid-teens. It was also a case of following the examples set by previous generations, as Waleed and Nora's parents had happily married their own cousins in shegar.

Neither sibling wanted this marriage and yet they did little to try and stop it.

"We're not the type of children who could say 'No' to their father," says Waleed.

They decided to surrender to what they saw as their destiny and give the marriages a chance. But it wasn't long before Waleed's relationship started to face problems.

After nine months, and against his family's wishes, he decided to divorce his wife.

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Waleed's in-laws, overcome with grief and anger, then decided to return his sister to her parents in retribution, ignoring the original agreement that the marriages would not depend on one another. And also ignoring the fact that Nora had turned out to be happy with her husband.

"Of course I felt guilty about my sister, she had to live away from her husband," Waleed says. But he insists he couldn't bear his unhappy marriage any more.

The dilemma of whether to choose your own happiness over your sibling's is just one of many complications couples face when entering this kind of marriage.

Fortunately, through the intervention of family and friends, Nora was reunited with her husband, but not all those who "swap marry" are as lucky.

Nadia is a case in point, and her pain and heartache will be familiar to many Yemeni men and women.

Listen to Yemen's Swap Marriages on the BBC World Service on Tuesday 29 July from 04:00 GMT